The technique of Orthogonal Frequency Division Multiplexing (OFDM for short) is a multi-carrier modulation communication technique in nature, and this technique is one of the core techniques in the 4th generation mobile communication (4G). In the frequency domain, the multi-path channel on OFDM system presents the characteristic of frequency selective fading. In order to overcome this kind of fading, in the frequency domain the channel should be divided into a plurality of sub-channels. The frequency spectrum characteristic of each of the sub-channels is nearly flat, and sub-channels of OFDM are mutually orthogonal; therefore, the frequency spectrums of the sub-channels are allowed to be mutually overlapped, thus being able to use the frequency spectrum resource to a large extent.
The technique of Multiple-Input Multiple-Output (MIMO for short) can increase the system capacity, improve the transmission performance, and converge with other physical layer techniques very well, thus becoming the key technique of beyond 3G (B3G) and 4G.
The Long Term Evolution (LTE for short) system and LTE-advance are important plans of 3rd generation partnership project. The OFDM technique and the MIMO technique are used. Simple introduction will be made about the LTE and LTE-A systems hereafter.
FIG. 1 and FIG. 2 are respectively schematic diagrams of a frame structure for LTE/LET-A in frequency division duplex (FDD for short) mode and in time division duplex (TDD for short) mode.
For the frame structure in FDD mode shown in FIG. 1, one radio frame having a length of 10 ms consists of twenty time slots of 0.5 ms in length which are numbered from 0 to 19; a subframe i of 1 ms in length consists of time slots 2i and 2i+1.
For the frame structure in TDD mode shown in FIG. 2, one radio frame of 10 ms in length consists of two half-frames of 5 ms in length, and one half-frame consists of five subframes of 1 ms in length. The subframe i consists of two time slots 2i and 2i+1 of 0.5 ms in length.
In the two kinds of frame structures, when a normal cyclic prefix (Normal CP) is used in the system, one time slot consists of seven uplink/downlink symbols; and when an extended CP is used in the system, one time slot consists of six uplink/downlink symbols. The above-mentioned symbols are OFDM symbols.
One resource element (RE for short) is a OFDM sub-carrier during one OFDM symbol interval, and a downlink resource block (RB) consists of 12 consecutive sub-carriers and 14 (12 in case of an extended cyclic prefix) consecutive OFDM symbols, wherein the RB is 180 kHz in the frequency domain, and is the duration of one time slot in the time domain, as shown in FIG. 3 (a 5M system). When resource allocation is performed in an LTE/LTE-A system, the allocation is performed with a resource block as a base unit.
The transmission scheme of the LTE/LTE-A system is as shown table 1 below:
TABLE 1The schematic table of the transmission scheme of theLTE/LTE-A systemSingle-antenna portDemodulated based on CRS ordemodulated based on DRSTransmit diversity schemeDemodulated based on CRSLarge delay CDD schemeDemodulated based on CRSClosed-loop spatial multiplexingDemodulated based on CRSMulti-user MIMO schemeDemodulated based on CRSDual layer schemeDemodulated based on UE specific RSUp to 8 layer transmissionDemodulated based on UE specific RS
Where the modes of Single-antenna port, Transmit diversity scheme, Large delay CDD scheme, Closed-loop spatial multiplexing, and Multi-user MIMO scheme, etc. support an MIMO application having four antennas to the greatest extent; therefore, cell common reference signal (CRS for short) is defined in protocol to serve for these transmission techniques. The common reference signal is sent in each non-MBSFN subframe. Corresponding antenna port #0, antenna port #1, antenna port #2 and antenna port #3 adopt the means of cell common reference signals (CRS) with full bandwidth. The function of the cell common reference signal comprises measuring the quality of the downlink channel and evaluating (demodulating) the downlink channel. If the cyclic prefix is a normal cyclic prefix, the positions of these common reference signals in a physical resource block are as shown in FIG. 4. If the cyclic prefix is an extended cyclic prefix, the positions of these common reference signals in a physical resource block are as shown in FIG. 5. In FIG. 4 and FIG. 5, the abscissa 1 represents the index of the OFDM symbol, and C1, C2, C3 and C4 correspond to logical port #0, logical port #1, logical port #2 and logical port #3 for the cell common reference signal.
It should be noted that there are some special cases when sending CRS, for example, in an MBSFN subframe, only the CRSs contained in the first two OFDM symbols needs to be sent; and in some newly introduced carrier types, no CRS need to be sent.
In addition, there are UE-specific reference signals, the reference signals are only transmitted in the time and frequency domains where the UE-specific physical downlink shared channel (PDSCH for short) is, mainly for supporting two closed-loop MIMO techniques of Dual layer scheme and Up to 8 layer transmission.
At present, no effective solution has been proposed aimed at that the sending of ePDCCH causes intra-cell interference in the control channels in related art yet.